The present Application relates to the cooling of hot continuous fibers using a coolant gas and has particular, but not exclusive, application to the cooling of Optical Fibers with helium. It provides both methods of cooling and apparatus for cooling.
The Optical Fiber industry faces increasing pressure to reduce manufacturing costs while maintaining the highest standards of product quality. A major part of the operating cost of an Optical Fiber production facility is taken up by utilities and, in an effort to reduce one expensive utility, attention has been directed to the cost of helium used as a heat exchange medium between the Optical Fiber as it passes vertically down the center of a draw tower and the externally cooled wall of the draw tower tube. In conventional Optical Fiber manufacture, all the helium supplied to the draw tower is allowed to escape to atmosphere through the open ends of the draw tower. It has been proposed in, for example, JP-A-60-046954, JP-A-4-240129 and EP-A-0094172 to recover and re-use the helium.
The level of technology involved in optical fiber manufacture is very high. The fibers are manufactured in a batch process with strict controls on diameter to micron levels. This requires a sophisticated level of control from the draw furnace, to fiber diameter measurement, fiber cooling, coating application and draw speed control. The optimum is to produce the longest single length of fiber with consistent diameter in the fastest possible time while maintaining the desired light transmissibility parameters. A significant challenge is found in starting the draw process; obtaining the maximum draw speed and stable operation as fast as possible. In view of the high level of control and the number of parameters that can effect the draw operation, recovering helium while maintaining stable operation presents a significant challenge. Changes affecting the efficiency of cooling in the draw tower can lead to off specification product, fiber breaks or coating problems which disrupt the fiber draw operation. It is economically sound to recover helium used in the draw process but not at the expense of increased production interruption and lost product. The final fiber product is worth much more than recovery of a utility gas, albeit an expensive one. Thus a stable, well defined, automated control system for helium recovery from an Optical Fiber manufacturing operation is required. This has not been satisfactorily achieved by the prior art proposals for helium reuse.
JP-A-60-046954 discloses a heat exchanger for cooling an Optical Fiber by contact with a helium or a helium/inert coolant gas mixture during passage through a passageway in a heat exchanger. The coolant gas is recirculated with addition of fresh helium and, when using a gas mixture, inert gas. The flow of the recirculated gas after said addition is controlled, in unspecified manner, by a valve and monitored by a flow meter. Optionally, the gas is cooled prior to entry into the passageway and/or the passageway is separately cooled.
JP-A-4-240129 acknowledges as prior art thereto a process of JP-A-60-046954 in which an Optical Fiber is cooled by direct contact with a helium/nitrogen gas mixture as it is passed through a heat exchanger passageway countercurrent to the direction of gas flow. The gas is recirculated with addition of fresh helium and nitrogen. The recirculated gas after said addition is cooled prior to entry into the passageway and/or the passageway is separately cooled. It is stated that a problem with the process of JP-A-60-046954 is the volume of helium required for cooling and the loss of helium gas and ingress of air at the ends of the heat exchanger passageway. JP-A-4-240129 seeks to overcome this problem by purifying the recycled helium to remove inter alia air which enters the heat exchanger passageway from the ends thereof. No details of flow control are provided.
In the embodiment of FIG. 1 of JP-A-4-240129, recycled helium gas is withdrawn from an upper outlet of the heat exchanger passageway and pumped to a helium gas purifier to remove air therefrom. Helium gas exiting the purifier is pumped to a gas mixer where make-up helium is added from a helium gas source. The resultant gas mixture is then supplied to a gas inlet at the bottom of the passageway.
The embodiment of FIG. 2 of JP-A-4-240129 differs from that of FIG. 1 thereof in that the wall of the heat exchanger passageway is cooled and the made-up recirculated helium is cooled prior to return to the passageway.
EP-A-0601601 discloses the recirculation of coolant gas from a heat exchanger having a passageway in which a hot continuous fiber is cooled with coolant gas and is particularly concerned with the cooling of Optical Fibers using helium. The flow of coolant gas into and out of the passageway is controlled based on one or more of the flow rate of, concentration of impurities in, and the pressure of recirculation coolant gas exiting the passageway. It is stated that the means for controlling the flow may be xe2x80x9cat least one flow resistance means, such as valves, orifices, sintered filters, narrow pipes having smaller diameters than the recovery conduit or packed bedsxe2x80x9d and that xe2x80x9cadjustment of the flow resistance means can be made manually or automatically based on the flow rate, pressure and/or composition of the coolant gasxe2x80x9d or xe2x80x9cthe flow resistance means can be preset or pre-adjusted based on experience and calculation or based on the flow rate, pressure and/or composition of a coolant gasxe2x80x9d. The only exemplification in EP-A-0601601 of monitored impurity in the coolant gas is that of oxygen concentration.
EP-A-0094172 discloses recirculation of nitrogen or other inert gas to and from a drier in which a solvent is evaporated. The flow of exhaust inert gas from the drier is controlled in response to the flow rate from the drier outlet and the oxygen concentration in the drier. The recirculated inert gas is pumped to a condenser unit where solvent is removed and returned in part to the main body of the drier and in part to the ends of the drier as inert gas curtains. If the gas outlet pressure falls below a predetermined level, the gas flow to the pump is made up with recirculated gas from the condensation unit. The distribution and amount of recycled gas and provision of make-up inert gas is controlled in response to the flow rate through the gas outlet; the flow rate through the gas inlet; the solvent concentration in the drier; the oxygen concentration in the drier; and the pressure in the recirculated flow from the condensation unit.
EP-A-0820963 discloses the recycle of helium from the fiber drawing step and at least one of the deposition and consolidation steps in an Optical Fiber manufacturing process. The used helium from these steps is partially purified to a low level purity and recycled to the fiber drawing process and/or further purified and recycled to at least one of the deposition, consolidation and fiber drawing steps. In an exemplified embodiment, provision is made to automatically vent purified recycle helium as a waste stream if the level of oxygen, chlorine, hydrogen chloride and moisture contaminants in the partially or fully purified recycle helium exceed predetermined levels.
WO-A-9749960 discloses the recycle of helium from the consolidation step in an Optical Fiber manufacturing process. The used helium from this step is either purified to a high level purity for recycle to the consolidation step or partially purified to a low level for usage in fiber draw or other process steps and subsequent purification for recycle to the consolidation and, optionally, other process steps. Helium from the fiber draw or other process steps can be purified and recycled independently of the recycle of helium to the consolidation step. In the exemplified embodiments, provision is made to automatically vent used or purified recycle helium to scrubbers if contaminant levels in the used or purified helium exceed predetermined levels.
It is the primary object of this invention to provide a relatively simple and effective system for reusing helium gas in Optical Fiber manufacture which will cause the minimum affect on application to an existing draw tower operation using fresh helium. More particularly, it is an object to provide such a system which will not reduce the length, consistency of diameter, or light transmissibility parameters of fiber produced or the speed of production compared with the use of only fresh helium.
The present invention provides an improvement in cooling a hot continuous fiber by simultaneously passing the fiber through an open ended passageway of a heat exchanger located in an ambient oxygen-containing atmosphere; introducing into the passageway a coolant gas comprising recirculated purified coolant gas and fresh coolant gas; allowing a portion of used coolant gas to leave the passageway at said open ends to limit ingress of ambient atmosphere into the passageway; removing, purifying and recirculating to the passageway a portion of the used coolant gas; and monitoring the oxygen content of said removed portion. The improvement is that the mass flow of coolant gas introduced into the passageway can be controlled independently of its composition by providing for automatic interruption of the removal of used coolant gas for purification and recirculation when the oxygen content of the used coolant gas exceeds a predetermined maximum concentration until such time as the oxygen content falls below a predetermined minimum concentration.
In one presently preferred embodiment, the method of the invention comprises simultaneously passing the fiber at constant speed through an open ended passageway of a heat exchanger located in an ambient moisture- and oxygen-containing atmosphere; introducing into the passageway a coolant gas containing at least about 95% helium and comprising recirculated purified coolant gas and fresh helium; allowing a portion of used coolant gas to leave the passageway at said open ends to limit ingress of ambient atmosphere into the passageway; removing, drying and recirculating to the passageway a portion of the used coolant gas; and monitoring the oxygen content of said removed portion, wherein said removal of used coolant gas for purification and recirculation is automatically interrupted when the oxygen content of the used coolant gas exceeds a predetermined maximum concentration and recommenced when the oxygen content falls below a predetermined minimum concentration; and the mass flow of coolant gas introduced into the passageway is maintained independently of its composition.
In a presently more preferred embodiment, the method of the invention comprises simultaneously passing the fiber at constant speed through an open ended passageway of a heat exchanger located in an ambient moisture- and oxygen-containing atmosphere; introducing into the passageway a coolant gas containing at least about 95% helium and comprising recirculated purified coolant gas and fresh helium; allowing a portion of used coolant gas to leave the passageway at said open ends to limit ingress of ambient atmosphere into the passageway; removing, drying and recirculating to the passageway a portion of the used coolant gas; and monitoring the oxygen content of said removed portion, wherein the mass flow of used coolant gas removed for purification and recirculation during normal operation is maintained at about 90% of mass flow of the coolant gas introduced into the passageway but said removal of used coolant gas is automatically interrupted when the oxygen content thereof exceeds a predetermined maximum concentration and recommenced when the oxygen content falls below a predetermined minimum concentration; said removal of used coolant gas is by a vacuum pump drawing used coolant gas from more than one said passageway and controlled to maintain a constant suction pressure; the mass flow of said removed gas portion from each passageway is regulated by a respective mass flow controller which is located upstream of said pump and regulates mass flow more rapidly than the vacuum pump can respond to corresponding changes in pressure levels; said interruption of flow from each passageway is by a shut-off valve located between said respective mass flow controller and pump; and the mass flow of coolant gas introduced into the passageway is maintained independently of its composition and is controlled by regulation of the pressure of said introduced coolant gas using respective pressure regulators on the fresh helium gas and the recirculated coolant gas set to provide recirculated coolant gas in preference to fresh coolant gas.
The invention also provides an apparatus for cooling a hot continuous fiber by the improved method of the invention. The apparatus comprises:
a heat exchanger for location in an ambient oxygen-containing atmosphere and having an open ended passageway to receive the hot continuous fiber for passage therethrough;
inlet means for introducing into the passageway a coolant gas comprising recirculated purified coolant gas and fresh coolant gas at a mass flow independent of said coolant gas composition;
outlet means for removing a portion of used coolant gas from the passageway;
purifying means for purifying said removed portion of used coolant gas;
pumping means for recirculating said removed coolant gas portion from said outlet means to said inlet means via said purifying means;
oxygen analysis means for monitoring the oxygen content of said removed coolant gas portion;
recirculation flow control means for interrupting said removal of used coolant gas; and
signal means for automatically activating said recirculation flow control means to interrupt said removal of used coolant gas when the oxygen content measured by said oxygen analysis means exceeds a predetermined maximum concentration and to recommence said removal when said measured oxygen content falls below a predetermined minimum concentration.
In a presently preferred embodiment, the apparatus of the invention comprises:
at least two heat exchangers for location in an ambient moisture- and oxygen-containing atmosphere, each exchanger having an open ended passageway to receive a respective hot continuous fiber for passage therethrough;
inlet means for individually introducing into each passageway a coolant gas comprising recirculated purified coolant gas and fresh coolant gas at a mass flow independent of said coolant gas composition and comprising respective pressure regulators on the fresh coolant gas and the recirculated coolant gas set to provide recirculated coolant gas in preference to fresh coolant gas;
respective outlet means for individually removing a portion of used coolant gas from each of the passageways;
purifying means for removing moisture from said removed portions of used coolant gas by freezing out the water content thereof;
pumping means for recirculating said removed coolant gas portions from said respective outlet means to said inlet means via said purifying means, said pumping means comprising a vacuum pump which is located upsteam of said purifying means, connected to the respective outlet means and controlled to maintain a constant suction pressure;
respective oxygen analysis means for individually monitoring the oxygen content of said removed coolant gas portions;
respective mass flow control means for individually maintaining constant during normal operation the ratio of the mass flow of used coolant gas removed by each outlet means to the mass flow of the coolant gas introduced into the respective passageway by said inlet means, each said mass flow control means comprising a mass flow controller upstream of said vacuum pump and regulating mass flow from the respective passageway more rapidly than the pump can respond to corresponding changes in pressure levels;
respective recirculation flow control means for individually interrupting said removal of used coolant gas from each passageway and comprising a shut-off valve located between said respective mass flow controller and said vacuum pump; and
respective signal means for activating each said recirculation flow control means to automatically interrupt said removal of used coolant gas from the respective passageway when the oxygen content measured by the respective oxygen analysis means exceeds a predetermined maximum concentration and to recommence said removal when said measured oxygen content falls below a predetermined minimum concentration.